Serratia marcescens, an opportunisic-pathogenic microorganism, has emerged in the Western hemisphere as a causative agent of serious, even fatal nosocomial infections. This organism has always been one of the most antibiotic resistant members of Enterobacteriaceae, the resistance being intrinsic. One of the most striking properties of this pigmented bacterium is the phenomenon of color instability. Some strains of Serratia undergo rapid variations in color. These variations occur at constant rates, in a sequence, and are reversible. They suggest the presence of DNA rearrangements similar to those that generate phase variation in Salmonella, pili variations in Neisseria gonorhoeae, antigenic variation in trypanosomes and immunoglobulin diversity in higher organisms. Serratia is also known to undergo a phase variation of its flagellar antigen. Variation of surface antigens is a phenomenon increasingly associated with pathogenesis and escape of a pathogen from the immune surveillance system of its host. We propose to investigate the nature of the color variation phenomenon. We propose to use transposon mutagenesis to explore the genetics of this organism. We propose to use various approaches to clone the region of the dNA responsible for producing color variation, and study its genetics. Besides elucidating the mechanism of color variation, this study will also help us understand how DNA rearrangements which are involved in regulating gene expression and invoked to explain rapid evolution and speciation, operate in other members of Enterobacteriaceae.